1. Field of the Invention
This invention relates to the recovery of oil from subterranean formations by chemical flooding methods.
2. Description of the Prior Art
Petroleum is frequently recovered from subterranean formations or reservoirs by permitting the natural energy of the reservoir to push the petroleum up through wells to the surface of the earth. These processes are referred to as primary recovery methods since they use the natural energy of the reservoir. However, a large amount of oil, generally in the range of 65-90% or more, is left in the subterranean formation at the conclusion of the primary recovery program. When the natural reservoir energy is unable to produce more petroleum, it is a common practice to resort to some form of supplemental recovery technique in order to recover additional petroleum left in the subterranean formation. These supplemental operations are normally referred to as secondary recovery operations. If this supplemental recovery operation is the second in a series of such operations, it will be referred to as a tertiary recovery operation. However, the terminology is unimportant for the purposes of this application and relates only to the sequence in which they are carried out.
The most widely used supplemental recovery technique because of its ease of implementation and low capital outlay is water flooding through injection wells drilled into the subterranean formation. In a water flooding operation, the injected fluid displaces oil through the formation to be produced from the injection well. A major disadvantage to water flooding, however, is its relatively poor displacement efficiency largely due to the fact that water and oil are immiscible at reservoir conditions, and high interfacial tension exists between the flood water and the oil. For this reason, after a water flood, a large portion of the oil is still left unrecovered in the reservoir.
It has been recognized by those skilled in the art that a solution affecting a reduction in this interfacial tension between the water and oil would provide a much more efficient recovery mechanism. Therefore, the inclusion of a surface active agent or surfactant in the flood water was recognized as an acceptable technique for promoting displacement efficiency of the reservoir oil by the water. For example, U.S. Pat. No. 3,468,377 discloses the use of petroleum sulfonates in water flooding operations and U.S. Pat. No. 3,553,130 discloses the use of ethylene oxide adducts of alkyl phenols for the same purpose. The use in water flooding operations of water soluble surface active alkaline earth resistant polyglycol ethers is disclosed in U.S. Pat. No. 2,333,381. Other specialized surfactants, as will be discussed later, have been discovered to have special properties useful in water flooding operations such as a tolerance for high salinity and calcium and/or magnesium ion concentrations often found in reservoir waters.
The surfactant systems, however, have been found to be deficient for certain situations. Surfactant solutions are often not viscous enough to provide a proper mobility ratio and, therefore, tend to finger through the oil instead of driving it ahead of a relatively flat front. The emulsification of a hydrocarbon with a surfactant often provides a material with increased viscosity over an aqueous surfactant solution and prevents fingering. The emulsions are variously called micellar dispersions, microemulsions and dispersions.
However, field operations employing these emulsions have not always been entirely satisfactory due to the fact that the surfactant component is often adsorbed by the formation rock to a relatively high degree, resulting in a destabilization of the emulsion as it progresses through the reservoir.